In a dual axis printer, with pin wheels used on one axis to regulate the amount of paper driven through the printer, it is impossible to drive exactly the same amount of paper with the platen, which is colinear with the other axis. This is because it is impossible to manufacture the platen with exactly the correct diameter. As a result, either more or less paper is driven at the platen paper drive.
If less paper is driven there, then paper tends to bunch up between the two axes and eventually jams. If more paper is driven there, and if paper cannot slip at the platen, then the tension builds between the axes until the paper tears.
Some printers feed less paper with the platen, in which case the excess paper is pushed past the platen by the pin feed drive. A problem with this method is that the paper tends to float slightly off the platen, making it difficult to control the spacing between the printhead and the paper. In addition, paper can be difficult to control when pushed, especially in a humid environment.
Other printers feed more paper with the platen, in which case the paper must slip at the platen as tension builds between the two axes. The problem with this method is that it is very difficult to control the amount of tension in the paper because it is very difficult to control the coefficient of friction between the paper and the platen, especially under a variety of environmental conditions. The effect on paper tension due to variations in the platen coefficient of friction is non-linear. Small percentage changes in the coefficient of friction will result in greater percentage changes in the paper tension.